Figure 10.

Two parallel thalamo-cortical circuits to auditory cortex in young (left) and aged (right) monkeys. Simplified schematic diagram of the ventral and dorsal thalamo-cortical connections to the internal and supragranular layers in the macaque auditory system. The illustration in to lower aspect represents the three main subdivisions of the medial geniculate, the dorsal (D), ventral (V), and medial (mc) divisions. The ventral pathway is from the ventral division to the middle cortical layers of A1 (left) whereas the dorsal pathway initiates in the dorsal division and projects to the upper layers of both A1 and CL. A1 also projects directly to CL. Requiring coincidence of these two pathways is one mechanism by which spatial tuning could be sharpened in CL relative to A1 in young monkeys. In aged monkeys, the increase of activity in A1 and CL degrades with the cortical dynamics that sharpen spatial tuning in CL relative to A1. Abbrevations: V, ventral division of the MGN; D, dorsal division of the MGN; mc, medial division of the MGN; A1, primary auditory area; CL, caudolateral belt area.